1. Field
Embodiments of the present invention relate to a method for calibrating a stage system, a stage system and a lithographic apparatus including such stage system.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In stage systems, such as lithographic apparatus stage systems, a position sensor may be applied to measure a position of the stage. Encoder-type measurement systems may be applied for such position measurement. Thereto, use may be made of a grid (also referred to as grating), such as a one or two dimensional grid, grid plate, etc, and a sensor head cooperating with the grid. Due to its manufacturing process, the grating may not be ideal and may incorporate deviations. Calibration of grid errors in the position sensor system may be performed by a so called “fishbone” technique, whereby a pattern is projected repetitively onto the substrate at different mutual locations, and afterwards the distances between the patterns are measured to compare them with the intended distances between the patterns. A measured difference relates to the error in the measurement system at the location of the exposure. By determining these differences over the complete working range of the position sensor, an error compensation value can be determined for a large number of locations on the measurement system, that, when applied, corrects the measurement system errors. Commonly, however not exclusively limited thereto, use is made of overlapping or adjacent patterns, such as line patterns, which may in some setups provide fishbone-like patterns. It will be understood that this calibration technique can however apply to any type of pattern.
The fishbone calibration technique may only be able to calibrate errors in the grid plate having a low spatial frequency, i.e. errors which change relatively gradually when moving the stage and grid plate relative to each other and having a spatial frequency in a frequency range related to the repetition pattern according to the fishbone technique or below. Inaccuracies or other effects that lead to errors at a higher spatial frequency may be difficult or impossible to detect by the above fishbone technique.